Functional Diversity in the Posidonia oceanica Ecosystem: an Example with Polychaete Borers of the Scales

Abstract

Species composition and spatio-temporal distribution of polychaete borers in Posidonia oceanica (L.) Delile scales, were studied in 4 stations located in two meadows off the Island of Ischia (Gulf of Naples) with different typology and at different depths: Castello at 1 m and 3–5 m depth, Lacco Ameno at 3–5 m and 22 m depth. Five species, all belonging to the family Eunicidae, were found as borers of scales: Lysidice collaris (Grube) (59% of the specimens collected), Lysidice ninetta Audouin and Milne-Edwards (32%), Nemotonereis unicornis Schmarda (8%), Palola siciliensis (Grube) and Marphysa fallax Marion and Bobretzky with only 2 specimens each. The spatio-temporal analysis showed the occurrence of polychaete borers (mainly the two species of Lysidice) at all stations and in all sampling months, except at the shallowest site (1 m) at the Castello meadow. The mean values of rhizomes affected by borers ranged from 19–21% at the shallow sites of both studied meadows (3–5 m) to 29% at the deep site (22 m) of the Lacco meadow. Traces of boring have been also frequently observed (37%) in the dead rhizomes, where specimens of 4 of the 5 recorded species were also found with a frequency of 23%. The mean number of individuals/m2 was 66±22 and 84±25 for the shallow sites (Castello and Lacco, respectively), and 49±15 for the deep site at Lacco. The temporal analysis did not show any particular pattern in borer colonisation, but relatively homogeneous abundances in all studied months. Boring of living le af bases (with detachment of the entire living shoot), and of the adjacent leaf tissues were sometime observed. The phenomenon of consumption of living tissues was very limited in the shallow meadows (Castello and Lacco 3–5 m, mean index of herbivory = 0.6% and 0.8% of the analysed rhizomes, respectively), while it was relatively more important in the deep site (Lacco 22 m, mean index of herbivory = 2.6%).The scales of P. oceanica represent an unique and preferential microhabitat for some boring polychaetes, whose colonisation resulted to be frequent, common to different meadows and depths and constant in time. The feeding and mechanical action of polychaete borers have strong ecological implications in enhancing scale fragmentation and decay by microbial activity, and in providing further microniches for other organisms (e.g. small polychaetes, sipunculids) that colonise the empty burrows. Last but not least the direct grazing on living plant tissues may have a direct impact on the system.